Teratocarcinomas from 120 Sv mice produce tumors in isogenic mice but not in BALB/c, C57B1/6 or C3H/He mice. A genetic analysis of tumor rejection indicates that the 129 Av tumors contain two transplantation rejection antigens called ter-1 and ter-2. The 129 Sv tumor expresses the ter-1b and ter-2b allele and the Balb/c mice have ter-1d and ter-2b, the C3H/He mice have ter-1k and ter-2b and the C57 B1/6 mice have ter-1 and ter-2 alleles. The d, k and x alleles may be similar or identical. The ter-1 locus maps 8% from H-2 on chromosome 17 on the centromeric or T-locus side of H-2. Experiments are in progress to prepare antisera to these antigens and detect them on the surface cells in culture.